The present invention relates to methods and apparatus for identifying a radionuclide in a liquid sample containing at least one unknown radionuclide.
While most samples presented to liquid scintillation counters consist of a single known radionuclide of which the activity is to be measured, occasionally the specific radionuclide is unknown or some doubt may exist concerning its identity. For example, labels on sample vials may be exchanged. In addition, it may be appropriate to confirm the identity of radionuclides in materials received from outside sources. Some knowledge of the identity of the primary radionuclide in waste materials would also be useful.
It has in most instances heretofore been possible to carry out experiments which produce results consistent with the presence of any one of three or four possible nuclides over a limited quench range. However, a method for the qualitative identification of a nuclide over a large quench range for a large class of nuclides would clearly be desirable.
It has been demonstrated that the identity of any one of a group of 59 nuclides could be determined, provided the sample being tested was unquenched [J. Mantel, Int. J. Appl. Rad. Isot. 23:407 (1972)]. The average energy of an unknown nuclide's spectrum was compared with a previously-calculated table of such values, and the unknown was identified as the radionuclide closest in value. This approach was clearly quite limited, as the only values available were those of unquenched samples.
U.S. Pat. No. 4,742,226 to de Phillipis, the entire disclosure of which is hereby incorporated by reference, describes a method for ascertaining the identity of an unknown radionuclide in a test sample by determining an external source quench-indicating parameter and a quench-indicating parameter for the test sample, comparing the observed values to a series of radionuclide equations determined for these parameters for radionuclides of interest, and identifying the radionuclide in the test sample by determining which radionuclide equation is satisfied by the observed values. Pursuant to the method of U.S. Pat. No. 4/742,226, the Spectral Index of a Sample (SIS) was related to a quench parameter provided by the Compton spectrum produced by an external standard. A Compton spectrum is produced by gamma radiation of a sample by a radionuclide outside the sample as an external standard. Electrons are scattered from the sample along with gamma photons of energy lower than the initial gamma radiation. Thus, EQU SIS=I+SQ.sub.ext
wherein I is the intercept and S the slope of a plot of SIS vs. the external standard quench parameter, Q.sub.ext. The three examples discussed in the patent (tritium, carbon-14 and chlorine-36) depended upon the specific external quench monitor tSIE, which is the transformed Spectral Index of the External Standard.
While this method is of some limited utility, it has several significant disadvantages. It requires the use of an external standard source and some mechanism for positioning the source adjacent the sample. Moreover, the methods of U.S. Pat. No. 4,742,226 require analysis of the resulting Compton spectrum in order to employ the aforementioned formula.
Accordingly, it would be clearly desirable to determine the identity of single radionuclides by methods and apparatus which do not require the use of external sources, source movement mechanisms and the associated analysis of the Compton spectrum. In addition, it would be highly advantageous if two or more spectral characteristics were measured and the resulting information used for confirmation of identity in those cases where there might be some ambiguity. Finally, it would be a significant advantage to provide a method for distinguishing between pairs of differently quenched nuclides (for example, H.sup.3 and Fe.sup.55 or C.sup.14 and S.sup.35) which are not distinguishable by heretofore known methods.